Working of alloys

ABSTRACT

A process for working superplastic sheet material comprising shaping the superplastic alloy by pressure or vacuum forming, annealing the sheet material before or after the pressure of vacuum forming, and bending at least a part of the shaped sheet at room temperature. In a preferred procedure rolled superplastic zinc/aluminium alloy sheet material is annealed at about 200* C for at least one hour, vacuum or pressure forming carried out at 200* C to 250* C after the annealing, and the shaped sheet allowed to cool to room temperature before bending.

United States Patent [191 Swanson Apr. 16, 1974 WORKING OF ALLOYS [75] Inventor: Colin John Swanson, Bristol,

England [22] Filed: Nov. 6, 1972 [21] Appl. No.: 303,718

[30] A Foreign Application Priority Data Nov. 4, 1971 Great Britain 51322/71 [52] US. Cl l48/l1.5 R [51] Int. Cl. C22f 1/16 [58] Field of Search t. 148/! 1.5 R

[56] References Cited UNITED STATES PATENTS 3,420,717 l/l969 Fields, Jr. et al. 148/11.5 R

3,567,524 3/1971 Cook et al. 148/1 1.5 R

Primary ExaminerW. W. Stallard Attorney, Agent, or Firm-Holman & Stern [5 7] ABSTRACT A process for working superplastic sheet material comprising shaping the superplastic alloy by pressure or vacuum forming annealing the sheet material before or after the pressure of vacuum forming, and bending at least a part of the shaped sheet at room temperature. In a preferred procedure rolled superplastic zinc/aluminium alloy sheet material is annealed at about 200 C for at least one hour, vacuum or pressure forming carried out at 200 C to 250 C after the annealing, and the shaped sheet allowed to cool to room temperature before bending.

10 Claims, No Drawings WORKING or ALLOYS This invention relates to the working of alloys which can be conditioned to exhibit the phenomenon known as superplasticity, and in particular to the working of 5 superplastic zinc/aluminium alloys.

The term superplasticity asused in this specification relates to the condition in which alloys exhibit a substantial strain rate sensitivity. Strain rate sensitivity, denoted m, is the exponential variable in the expression a KE'", wherein or represents stress in load per unit area, g represents strain rate in terms of length change per unit time, and K represents a constant which is termed the strain rate co-efficient.

While the existence of a substantial strain rate sensitivity m determines the ultimate elongation to which an alloy may be subjected without fracture, the strain rate coefficient K is significant in determining the strength of the material and hence the amount of work which must be put in to achieve a given forming operation.

It is already known to condition a blank of alloy to exhibit su'perplasticity by means of a heat treatmentlquenching/working technique and to cause the blank thus conditioned to conform to the shape of a moulding surface by means of fluid pressure or vacuum. However, after this moulding operation the alloy may still require further working to produce a further shape feature, for example a flange may be required on the finished article and this may involve a bending operation on at least a part of the moulded shape.

It may be demonstrated that the desirable properties of superplastic alloys may be achieved only at the expense of a reduction in the capability of the material to withstand bending stresses without cracking.

Because superplastic alloys are characterized by their high degree of strain rate sensitivity it is clear that difficulties encountered during cold bending may be overcome by lowering the rate of such bending. While this expedient may be acceptable in limited instances it is unlikely to be acceptable in a commercial forming process. v

The invention in one aspect consists in a process for working alloy sheet material which has been conditioned to exhibit superplasticity, comprising: shaping the superplastic alloy by pressure or vacuum forming; annealing the sheet material before or after the said pressure or vacuum forming; and bending at least a part of the shaped sheet at or near room or ambient temperature. Room or ambient temperature is to be understood to be about 20 C.

Usually the annealing is carried out before the vacuum or pressure forming operation.

Preferably the annealing temperature is within the range 150 to 250 C, more preferably about 200 C.

The annealing time is suitably l to 2 hours.

In a preferred procedure rolled superplastic zinc- /aluminium alloy sheet material is annealed at about 200 C for at least 1 hour, vacuum or pressure forming carried out at 200 C to 250 C, and the shaped sheet allowed to cool to room temperature before bending.

The invention in another aspect consists in a process for working an alloy sheet material comprising rendering the alloy superplastic (as hereinbefore described), shaping the superplastic alloy by pressure or vacuum forming, annealing the sheet material before or after the said pressure or vacuum forming, and bending at least a part of the shaped sheet at or near room or ambient temperature.

The invention will be further described with reference to the following examples.

Tables 1, 2 and 3 below show the results of bending strips of a zinc/aluminium alloy (containing 87% by weight of zinc, 22% aluminium, and 0.15% of copper as a ternary element) of dimensions 1% inches X 3% inches around different bend radii (in inches) for three different gauges of alloy sheet, i.e. 0.025 inches, 0.050 inches, and 0.075 inches gauge. The specimens were annealed for 1 hour at 200 C and 250 C and then bent cold around the specified radii. The results of the cold bend tests are shown in the Tables.

The rating of the effect of bending is related to the following scale:

Rating Degree of Cracking Complete fracture Heavy cracking Moderate cracking Light cracking No cracks.

CQUIU-lv- Table 1 0.025 inch Gauge Sheet Bend As Rolled Y Radius (at room Anneal (Inches) temperature) Temperature (C) Table 2 0.050 inch Gauge Sheet Bend As Rolled Radius (at room Anneal (l nches) temperature) Temperature Table 3 0.075 inch Gauge Sheet Bend As Rolled Radius (at room Anneal (lnches) temperature) Temperature (C) Table 4 Effect of temperature Bend Radius As Rolled Annealing Temperature (inches) (at room 100 I50 200 250 temperature) .250 l l 8 9 9 .l25 l l 7 9 9 .080 l l l 6 8 .040 l l l 4 7 .020 l l l 4 7 0 l 1 l 4 7 It will be recognised that to some extent the improvement in cold bending which is achieved by prior annealing arises from grain growth in the superplastic alloy. Increasing grain growth detracts from the superplastic properties. However, annealing for one hour at 200 C is likely to have a negligible effect on such superplastic properties.

I claim:

1. A process for working alloy sheet material which has been conditioned to exhibit superplasticity, comprising: taking the said alloy sheet material; causing the said sheet material to conform to a moulding surface to shape the alloy sheet material; annealing the sheet material after the said shaping; and bending at least a part of the shaped sheet at room temperature.

2. A process as claimed in claim 1 wherein the annealing temperature is within the range to 250 C.

3. A process as claimed in claim 1 wherein the annealing time is from 1 to 2 hours.

4. A process as claimed in claim 1 wherein the material being worked is superplastic zinc/aluminium alloy sheet material.

5. A process for working alloy sheet material which has been conditioned to exhibit superplasticity, comprising: taking the said alloy sheet material; annealing the said sheet material; causing the sheet material to conform to a moulding surface to shape the alloy sheet material after the said annealing; and bending at least a part of the shaped sheet at room temperature.

6. A process as claimed in claim 5 wherein the annealing temperature is within the range 150 to 250 C.

7. A process as claimed in claim 5 wherein the annealing time is from 1 to 2 hours.

8. A process as claimed in claim 5 wherein the material being worked is superplastic zinc/aluminium alloy sheet material.

9. A process for working alloy sheet material which has been conditioned to exhibit superplasticity, comprising: taking the said alloy sheet material; annealing the said sheet material at about 200 C for at least 1 hour; causing the said sheet material to conform to a moulding surface to shape the alloy sheet material after the said annealing, the said shaping being carried out at a temperature of from 200 to 250 C; allowing the shaped sheet to cool to room temperature; and bending at least a part of the shaped sheet at room temperature.

10. A process as claimed in claim 9 wherein the material being worked is superplastic zinc/aluminium sheet material. 

2. A process as claimed in claim 1 wherein the annealing temperature is within the range 150* to 250* C.
 3. A process as claimed in claim 1 wherein the annealing time is from 1 to 2 hours.
 4. A process as claimed in claim 1 wherein the material being worked is superplastic zinc/aluminium alloy sheet material.
 5. A process for working alloy sheet material which has been conditioned to exhibit superplasticity, comprising: taking the said alloy sheet material; annealing the said sheet material; causing the sheet material to conform to a moulding surface to shape the alloy sheet material after the said annealing; and bending at least a part of the shaped sheet at room temperature.
 6. A process as claimed in claim 5 wherein the annealing temperature is within the range 150* to 250* C.
 7. A process as claimed in claim 5 wherein the annealing time is from 1 to 2 hours.
 8. A process as claimed in claim 5 wherein the material being worked is superplastic zinc/aluminium alloy sheet material.
 9. A process for working alloy sheet material which has been conditioned to exhibit superplasticity, comprising: taking the said alloy sheet material; annealing the said sheet material at about 200* C for at least 1 hour; causing the said sheet material to conform to a moulding surface to shape the alloy sheet material after the said annealing, the said shaping being carried out at a temperature of from 200* to 250* C; allowing the shaped sheet to cool to room temperature; and bending at least a part of the shaped sheet at room temperature.
 10. A process as claimed in claim 9 wherein the material being worked is superplastic zinc/aluminium sheet material. 